ACAB isn't a general communist position. Communists have always implemented policing in every state they run. ACAB is a strategic position to hold against cops whose primary function under a capitalist state is to protect the interrsts of private property.

As part of the state, we want it to eventually wither away. But to immediately abolish all cops after a revolution is more of an anarchist position (though anarchists have historically also still had people deputized to do cop things).